set QuakeML-valid resource identifiers on init

I think we should set a QuakeML-valid default Resource Identifier for all event objects that have one associated with them. I think many people that use the event classes will eventually output to QuakeML and might not really care about explicitly handling resource identifiers themselves. By changing this, event type objects could be linked against each other right after initialization without the need to fiddle with resource identifiers.
Tests would have to be adapted.. What do you think @krischer?